The present invention relates to an electromagnetic drive control device for controlling electromagnetic driving devices which are employed in various instruments as a drive source.
An electromagnetic driving device typically drives an actuator making use of an electromagnetic force generated between a permanent magnet and a coil in which an electrical current flows. The driving force can be controlled by controlling the current flowing through the coil. Since the electromagnetic driving device can be made relatively compact in size, it is widely employed, as a driving source, for an objective lens driving device of a camera, a scanning position compensating device for a laser scanning device, a driving device for a linear motor car, and the like.
As an example, the scanning position compensating device will be described. In the scanning position compensating device for a laser scanning device, a driving coil is swingably (rotatably) supported in a magnetic field generated by a magnet which is secured to the electromagnetic driving device. As the electrical current is fed to the driving coil, it swings due to the electromagnetic force. The driving coil typically supports a prism, which swings with respect to the optical axis as the driving coil swings, thereby deflecting a passage of the laser beam. In this type of drive control device for controlling the electromagnetic drive that controls the direction of the electrical current flowing through the coil, a circuit as shown in FIG. 9 is generally employed.
The circuit shown in FIG. 9 includes a drive circuit 41, which includes an operational amplifier OP, a resistor R, and a current buffer circuit 411. The current buffer circuit 411 is configured such that an NPN transistor TR1 and a PNP transistor TR2 are connected in accordance with a complimentary emitter follower connection. The drive circuit 41 is a so-called voltage-current conversion circuit, which outputs an electrical current in accordance with a voltage of an input drive control signal CS to a drive circuit 30.
In such a voltage-current conversion type drive control circuit, the output current I is grounded through a drive coil 24 and the resistor R. The drive circuit 41 operates such that the voltage R*I equals the drive control signal CS. When the drive control signal CS is positive, a positive voltage +Vcc is applied to an terminal A of the drive coil 24, and thus, the current flows from the terminal A to a terminal B. When the drive control signal CS is negative, a negative voltage xe2x88x92Vcc is applied to the terminal A, thereby the electrical current flowing from the terminal B to the terminal A. As the direction of the electrical current flowing through the drive coil 24 switches as described above, the direction of the electromagnetic force caused between the drive coil 24 and the magnet 223 switches. Thus, the drive coil 24 can be driven to operate as desired. Further, depending on the voltage of the drive control signal CS, the voltage output by the drive circuit 41 varies. Then, the current flowing through the drive coil 24 varies, and the electromagnetic force between the drive coil 24 and the magnet 223 varies. Accordingly, by controlling the voltage of the drive control signal CS, the amount of the swing movement of the drive coil 24 can be controlled.
In the conventional drive control circuit as described above, when power sources (i.e., +Vcc and xe2x88x92Vcc) are turned from ON to OFF and the voltages change from 0V to designated values (+Vcc and xe2x88x92Vcc), one of the power sources may reach the designated voltage earlier than the other. In such a case, the performance of the circuit may become unstable. In a particular case, the output of the operational amplifier OP is fixed, for example, to +Vcc or xe2x88x92Vcc. In such a case, a maximum (or minimum) drive current is output from the drive circuit 41 to the drive coil 24. Then, an electrical damage and/or a mechanical damage of the electromagnetic drive device will be caused.
Further to the above, when the power sources are in OFF condition, the following problem may occur. When the power sources are in OFF condition, no electrical current flows through the coil 24. Since the drive coil 24 is in an electrically open status, no electromagnetic force is generated between the drive coil 24 and the magnet 223 when the power sources are in the OFF condition. If an oscillation or a shock is applied from outside to the drive coil 24 under such a condition, the drive coil 24 may be swung greatly exceeding a limited movable range. In such a case, thin feed lines connected to the drive coil 24 may be cut, or a supporting mechanism for the drive coil 24 may be mechanically damaged.
As described above, the conventional drive control device provided with two power sources has defects.
It should be noted that a drive control device employing a single power source also has a similar problem, if a relatively long time is required till the voltage of the power source reaches the designated value after turning ON the power source. In such a case, the maximum current may flow through the drive coil and the electromagnetic drive device may be electrically damaged when the power source is turned ON. Further, since the coil is in the unstable condition when the power source is in the OFF condition, the electromagnetic drive device may be mechanically damaged due to the external oscillation or shock.
As explained above, in the conventional electromagnetic drive control device, the operation of the electromagnetic driving device may be unstable, and the life thereof may be relatively short.
In view of the above problems, it is an object of the present invention to provide an improved electromagnetic drive control device for an electromagnetic driving device, in which the above-described problems when the power sources are turned ON and/or when the power sources are in the OFF condition are resolved.
For the above object, according to the invention there is provided a drive control circuit for an electromagnetic driving device including a magnet and a drive coil that moves due to an electromagnetic force, when an electrical current flows therethrough. The drive control circuit may include a drive circuit that feeds an electrical current to the drive coil, the drive circuit including at least one voltage source, a short-circuit system that short-circuits the drive coil, the short-circuit system releasing the short-circuited condition of the drive coil in accordance with an output voltage of the at least one voltage source.
With this configuration, when the voltage source is in the OFF condition, since the drive coil is short-circuited, a counter electromotive force is generated when the external shock or oscillation is applied, which prevents the excessive movement of the drive coil. Further, when the voltage source is turned ON, the output current of the drive control circuit will be or will not be fed to the drive coil depending on the output voltage of the voltage source. Thus, the above-described problem of the overcurrent across the drive coil can be prevented.
Optionally, the short-circuit system may include a voltage detection circuit that detects the output voltage of the at least one voltage source.
Still optionally, the short-circuit system may include an electromagnetic relay system, which is provided with a magnet coil actuated in accordance with an output of the voltage detection circuit, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated.
Further optionally, the voltage detection circuit may include a switching circuit connected between the at least one voltage source and the magnet coil, the switching circuit being turned ON to connect the at least one voltage source and the magnetic coil when the output of the voltage source has satisfied a predetermined condition.
Still optionally, the drive circuit may have an input terminal to which a control signal is input, the drive circuit outputting an electrical current to the drive coil through the short-circuit system.
In a particular case, the at least one voltage source includes a positive voltage source and a negative voltage source. In this case, the short-circuit system may include a first voltage detection circuit that detects the output voltage of the positive voltage source and a second voltage detection circuit that detects the output voltage of the negative voltage source, and the short-circuit system may maintain or release the short-circuited condition of the drive coil in accordance with the output voltages of the positive and negative voltage sources.
According to one embodiment, the short-circuit system releases the short-circuited condition of the drive coil when the absolute values of the output voltages of the positive and negative voltage sources exceed predetermined values, respectively.
In this case, the short-circuit system may include an electromagnetic relay system which is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated. The voltage detection circuit may include a first switching circuit connected between the positive voltage source and the one end of the magnet coil and a second switching circuit connected between the negative voltage source and the other end of the magnet coil, the first and second switching circuits being turned ON when the absolute values of the output voltages of the positive and negative voltage sources exceed the predetermined values, respectively.
According to another embodiment, the short-circuit system releases the short-circuited condition of the drive coil when a difference between the output voltages of the positive and negative voltage sources exceeds a predetermined value.
In this case, the short-circuit system includes an electromagnetic relay system which is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated. The voltage detection circuit may include a first switching circuit connected between the positive voltage source and the one end of the magnet coil and a second switching circuit connected between the negative voltage source and the other end of the magnet coil, the first and second switching circuits being turned ON when the difference between the output voltages of the positive and negative voltage sources exceeds the predetermined value.
According to a further embodiment, the at least one voltage source includes a single voltage source, the short-circuit system includes a single voltage detection circuit that detects the output of the single voltage source, and the short-circuit system maintains or releases the short-circuited condition of the drive coil in accordance with the output voltages of the single voltage sources.
In this case, the short-circuit system releases the short-circuited condition of the drive coil when the output voltages of the single voltage sources exceed a predetermined value.
Further, the short-circuit system includes an electromagnetic relay system is provided with a magnet coil, a contact switch provided between both end terminals of the drive coil, the contact switch neutrally connecting the both end terminals of the drive coil, the contact switch disconnecting the both end terminals of the drive coil when the magnet coil is actuated, and the voltage detection circuit includes a single switching circuit connected between the single voltage source and one end of the magnet coil, the switching circuit being turned ON when the output voltages of the single voltage sources exceeds the predetermined value.